Ammonites

Orthocone nautiloid

Orthocones aren't ammonites, but they are early relatives, so I've included them here. Orthocone just means straight-shelled, and these tend to be called Orthoceras, although there is some debate over whether that's its correct genus name.

These are preserved in the marble of the High Atlas, Morocco, and the stark contrast of black and white details their chambers nicely. 380 million years old.

13cm